To explore the effects and possible mechanisms of environmental cadmium (Cd) on the mitochondrion structure and cellular energy metabolism of Pelteobagrus fulvidraco gill, specimens of P. fulvidraco were exposed to different Cd concentrations (0, 50, 500 microg x L(-1) freshwater) for 7 days. The results showed that when exposed to 50 microg Cd x L(-1), the mitochondrion structure of P. fulvidraco was intact, and no significant differences were observed in all test biochemical indicators, compared with the control (P > 0.05). But, when exposed to 500 microg Cd x L(-1), mitochondria were damaged severely, and except that the LD, MDA, ADP and AMP contents in mitochondria as well as the K+ concentration in plasma were significantly higher than the control, all other test biochemical indicators were decreased significantly (P < 0.05). It was suggested that short-term exposure to higher concentrations of Cd would lead to the obstruction of energy supply and utilization due to the inhibition of PFK and ATPase activity and the oxidative damage of mitochondria resulted from the decrease of SOD activity, and decrease the ability of gills in regulating plasma ion composition and osmolality, which could be one of the possible mechanisms for Cd toxicity.